In known keyboard type electronic musical instruments wherein the output can be varied by varying the strength with which the keys are struck, an induced voltage due to the interaction of a coil and a magnet is utilized, or the time needed for switching the key switch which is operated by the key being struck is utilized. An example of the latter system will be described with reference to FIG. 1.
FIG. 1 shows a well-known circuit for a keyboard type electronic musical instrument in which a key 5 having a transfer rod 4 is adapted to be moved vertically by a force represented by the arrow to switch the movable contact 1 of switch S from the break contact 2 to the make contact 3. A parallel connected capacitor 6 and a resistor 7 constituting a time constant circuit are connected to the movable contact 1, and a gate circuit 9 is adapted to be actuated to gate the output of an original tone oscillator 10 by an envelope signal from a generator circuit 8 connected to the make contact 3, so that the output waveform from the oscillator is supplied to the output terminal 11. A D.C. power source E' is connected to contact 2. If the key 5 having the transfer rod 4 thereon is displaced in the direction of the arrow by a player, the movable contact 1 of the keyswitch S will be transferred from the break contact 2 on which a D.C. potential of the source E' is impressed, to the make contact 3. The transfer speed of the contact 1 is proportional to the speed of movement of the key 5 being struck. At the instant the movable contact 1 of the keyswitch S leaves the break contact 2, the charge on the capacitor 6, which has been charged by the D.C. source E' up to that moment, will begin to discharge through the resistor 7 so that the potential across the capacitor 6, which is supplied to the make contact 3 of the keyswitch S, will have a magnitude which is reduced in proportion to the exponential function of the time necessary for the completion of movement of the keyswitch S. In other words, the make contact 3 of the key switch S will be supplied with a greater voltage for higher speed of the key being struck than for a lower speed thereof. The output signal of the envelope signal generating circuit, which is driven by the voltage applied to the make contact 3, is introduced into the gate circuit 9, whereby the output of the original tone oscillator is transformed into a signal, the magnitude of which is substantially proportional to the speed with which the key is struck, so that a musical tone signal is taken out from the output terminal 11.
However, if the circuit of FIG. 1 is to be utilized in an electronic musical instrument which can be played so as to produce tones similar to a piano, a damper device is necessary in addition to the elements described above. That is, in an ordinary, mechanical piano, if the sustain pedal is activated, the damper means will be moved so as to be free of the strings, whereby the tone created by the key being struck will be sustained, even if the key is restored to its initial position, and the tone will only be extinguished along a predetermined damping curve. But means are provided by which, when the pedal is not actuated, the tone will die away at the same time as the key is restored to its initial position.
Such a damper also plays a very important role in an electronic piano. To provide the circuit according to FIG. 1 with a sustain effect, another damper circuit having a switch as a part thereof would be necessary. However, in a practical musical instrument having a plurality of keys, the provision of added switches for the respective key circuits to attain a sustain effect would lead to considerable disadvantages, such as an increase in the number of parts for such circuits, a higher rate of the occurrence of faults, a higher cost of the instrument, and difficulty in gating tone signals by the envelope signals without damaging the tone signal envelope.